Tsunami, a devastating wave having enormously destructive power, causes nearly unrecoverable loss for the islands and the cities along the coast that are destroyed by its frightening force. Therefore, there has long been an effort to construct a sound tsunami alert system and to increase the alert time as long as possible before the tsunami reaches land in order to provide the public sufficient time to prepare in order to reduce the number of casualties and loss of property. The 2004 Sumatra Tsunami disaster was the biggest in scale and caused the highest number of casualties in recorded history.
The features of tsunami are the extremely long wavelength and the very fast propagation speed, which are about 200 km and 600 km per hour, respectively. Besides, the amplitude of tsunami in the deep ocean is about one meter only, so that it is hard to be noticed and detected. In the global sea level observing system (GLOSS), tide gauges are deployed along the coast or around the islands to monitor the sea level to detect possible occurrence of tsunami. Additionally, in the deep-ocean assessment and reporting of tsunami system (DART), sensors are anchored on the sea floor and connected to the buoy on the sea surface, and the data collected from both are transmitted from the buoy to the satellite and then to the central station. When a shockwave of the sea floor or a wavefront of the tsunami passes through the sensor or the buoy, it is possible to detect whether a tsunami or an earthquake occurs.
However, it is always too late to sense the coming of the tsunami via the above methods, and inhabitants along the coast still do not have sufficient time to evacuate. Besides, the cost to build, configure and maintain these equipments is highly expensive.
After the disaster of the 2004 Sumatra Tsunami, scientists discovered that there is strong and irregular variation in the ionosphere when an earthquake or a tsunami happens. This is because the acoustic waves and gravity waves caused by tsunami will propagate upward into the ionosphere, which results in the variation of electron density. Such variation occurs immediately after the tsunami passes through. Therefore, measurement and observation on the total electron content of the ionosphere is an effective way to detect whether a tsunami occurs.
Presently, techniques of using satellites to measure the electron density in the ionosphere have been proposed. The National Aeronautics and Space Administration (NASA) has demonstrated a global differential GPS (GDGPS) system, wherein the real-time global maps of ionospheric electron density can be generated per five minutes. Moreover, the Jet Propulsion Laboratory (JPL) uses the data collected from land stations all over the world to generate real-time global maps of ionospheric total electron content per five minutes, and provides them to the single-frequency GPS users. In addition, there are many organizations, institutions and network systems which can provide data collected from GPS for extensive scientific applications, such as International GPS Service (IGS), Japan GPS Earth Observation Network System (GEONET), Center for Orbit Determination in Europe (CODE) and US NOAA Space Environment Center, etc.
Possible factors to cause perturbation of electron density include season change, sunspot activities, geomagnetic storm, man-made atmospheric pollution, as well as earthquakes and tsunamis mentioned above. By using the International Reference Ionosphere (IRI) model, the perturbation of ionosphere under normal condition can be inferred, thus various kinds of irregularities of ionosphere can be forecasted reliably. In addition to the man-made factors, however, after the atmospheric gravity waves generated by tsunami propagate to the ionosphere, the reconstructed image of variation of electron density thereof through computer simulation will show different features from those under normal condition.
In order to overcome the drawbacks in the prior art, a tsunami detection method and system are provided. The particular designs in the present invention not only solve the problems described above, but also are easy to be implemented. Thus, the present invention has the utility for the industry and the government.